Bacteria that are able to persist in hosts to cause long term infections are, by definition, able to evade host immune defenses. While establishment of persistent infection can benefit the bacteria, in certain cases where the bacteria itself causes little damage, allowing the establishment of prolonged infection may be more beneficial to the host than continued attack by the immune system. Borrelia burgdorferi, the causative agent of Lyme disease, produces no toxins or other virulence factors thought to damage the host. Instead, most of the manifestations of Lyme disease are thought to be the result of the immune response to the organism. In its natural Peromyscus mouse host, little or no reaction to the organism is typically seen despite the fact that, once infected, the organism persists for the life of the animal. In humans and inbred mice (which do develop immune responses to the organism), inflammation is thought to be initiated by receptors of the innate immune system. In vitro, loss of receptors of the innate immune system that recognize B. burgdorferi such as toll-like receptor 2 (TLR2) or Nod2 results in a decrease inflammatory response. However, in vivo studies of animals deficient in these receptors or their adaptor molecules have not reduced inflammation and in many cases, have actually resulted in increased inflammation. We hypothesize that the host has evolved methods for dampening the immune response over time and that during the course of prolonged exposure, these receptors play a more important role in triggering innate immune tolerance, rather than activating acute inflammatory pathways. In preliminary studies, we have shown that exposure of macrophages to B. burgdorferi can reduce release of inflammatory cytokines and increase release of anti-inflammatory cytokines upon re-exposure to the organism. In this proposal, we will first identify pathways and mechanisms involved in mediating innate immune tolerance to B. burgdorferi. We will study both pathways that have been identified in playing a role in tolerance to other agents (e.g. lipopolysaccharide) as well as perform unbiased studies to identify new mechanisms. In Aim 2, we will develop animal models to study the importance of innate immune tolerance in Lyme disease. We will test both ex vivo and in vivo systems for isolating the role of different pathways during B. burgdorferi infection. Successful completion of these experiments will provide proof of principle for a new strategy of host defense (mutualism or ?benign neglect?) against organisms that are considered pathogens and establish a rationale for the existence of innate immune tolerance.